The immunology of human breast cancer

Over recent years, immunotherapy with immune checkpoint inhibitors (ICIs) has revolutionized the clinical management of various solid neoplasms, including melanoma and lung carcinoma. A proportion of these malignancies exhibit a baseline immunological configuration that can be successfully targeted with ICIs to support durable clinical responses. Conversely, breast neoplasms other than the triple-negative subset respond poorly to ICIs, reflecting numerous tumour-intrinsic and microenvironmental barriers against effective anticancer immunity. Notably, most hormone receptor-positive breast cancers tend to have limited immunogenicity and to establish a local and systemic immunosuppressive microenvironment that hinders responses to ICIs. Here, we summarize the main immunological features of human breast tumours, whenever possible comparing across disease subtypes, stages and treatment outcomes. Understanding the complex interplay between breast cancer, its hormonal regulation and the immune system will be essential for unlocking the full potential of ICIs and other immunotherapies in this oncological indication.